Roll Observatory packages and add a roll script

packages/polymer_interop/lib/src/build/replace_polymer_js.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 library polymer_interop.src.build.replace_polymer_js.dart;
 
 import 'dart:async';
 import 'package:barback/barback.dart';
 
 /// Replaces `polymer.min.js` with `polymer.js` in development mode.
 class ReplacePolymerJsTransformer extends Transformer {
   BarbackSettings settings;
 
   ReplacePolymerJsTransformer.asPlugin(this.settings);
 
   bool isPrimary(AssetId id) {
     if (settings.mode == BarbackMode.RELEASE) return false;
     return id.path == 'lib/src/js/polymer.html';
   }
 
   Future apply(Transform transform) async {
     var contents = await transform.primaryInput.readAsString();
     contents = contents.replaceFirst('polymer.min.js', 'polymer.js');
     transform
         .addOutput(new Asset.fromString(transform.primaryInput.id, contents));
   }
 }